1. Field of the Invention
The present invention relates to hydraulic accumulators in general and, in particular, hydraulic accumulators for use in hydraulic drive trains of hybrid motor vehicles.
2. The Prior Art
Hydraulic hybrid vehicles utilize accumulators to store mechanical energy, either energy recovered from braking the vehicle or excess energy generated by the engine. See U.S. Pat. No. 5,495,912 and U.S. patent application Ser. No. 09/479,844 for details of the use of accumulators in hydraulic hybrid vehicles. However, conventional hydraulic hybrid vehicles suffer from a problem with permeation of compressed gas in the accumulator (that gas permanently “trapped” within a bladder) through the barrier (bladder) that separates the compressed gas from the working fluid liquid (i.e., “hydraulic fluid” or gas).
Conventional accumulators are made in several designs including: piston accumulators where the piston in a cylindrical accumulator vessel separates the hydraulic fluid from the gas (usually nitrogen), the latter being compressed to store energy by hydraulic fluid flowing into the vessel, bladder accumulators which use an elastic bladder to separate the hydraulic fluid from the gas, and diaphragm accumulators which use a diaphragm to separate the hydraulic fluid from the gas.
The most common accumulator design, and the preferred design for accumulators used in hydraulic hybrid vehicles, is the bladder accumulator. FIG. 1 shows a cross-section of a conventional bladder accumulator. An elastic bladder 11 contains compressed gas. Gas is charged through bladder fixture 12. Hydraulic fluid (liquid) is contained in space 13 and enters and exits through fixture 14. Anti-extrusion valve 15 prevents the bladder from being extruded as the liquid volume approaches zero. The bladders used in these accumulators are made of an elastomer (such as nitrile rubber). Although the pressure in the hydraulic fluid on one side of the bladder is the same as the pressure of the compressed gas on the other side of the bladder (during normal operation), molecules of the compressed gas permeate through the bladder and dissolve in the hydraulic fluid trying to reach an equilibrium concentration. The high pressures utilized in the high pressure accumulator facilitate a rather high equilibrium quantity of dissolved gas. When the hydraulic fluid is discharged to drive a hydraulic motor, the liquid pressure drops as the hydraulic fluid flows through the motor producing mechanical power. The pressure drop (for example, from 5000 psi to 100 psi) results in a low pressure liquid being discharged from the motor and then being routed to the low pressure accumulator for storage until needed during regenerative braking or during engine operation, where the engine pump receives liquid from the low pressure accumulator and discharges high pressure liquid to a hydraulic motor and/or to the high pressure accumulator for storage until again needed. The low pressure liquid leaving the hydraulic motor has (i.e., can contain) a much lower equilibrium quantity of dissolved gas, so a significant quantity of dissolved gas is expelled and the low pressure liquid and small bubbles of gas flow to the low pressure accumulator (or the engine driven pump if operating). In the low pressure accumulator the gas bubbles rise to the top of the liquid level and a pocket of gas forms. This separate gas volume causes several problems, including: (1) the separate gas volume on the liquid side of the bladder in the low pressure accumulator displaces liquid and reduces the effective capacity of the accumulator system and (2) entrained gas which is discharged with liquid when needed by the hydraulic pump causes the pump to experience cavitation and erratic torque fluctuations which are unacceptable.
The elastic bladder material can be chosen to minimize permeation, but the nature of an elastomer is such that permeation cannot be eliminated or even minimized sufficiently to be acceptable for the preferred closed system in a hydraulic hybrid vehicle. Permeation resistant, flexible coatings such as polyvinyl alcohol can be used on the gas side of the bladder, but even with such coatings the permeation level is still unacceptable.
Accordingly, it is an object of the present invention to substantially eliminate permeation of gas into the hydraulic fluid in a hydraulic accumulator.
Another objective of the present invention is to provide for venting any permeated gas from the hydraulic fluid.